Diffusion Potentials and Potassium Distribution across the Gallbladder Wall.

During the process of concentration of bile by the gallbladder, water and electrolytes are absorbed, leaving behind bile acids, bilirubin pigment, and other substances to which the gallbladder epithelium is not permeable. It has recently been postulated (1) that this resorptive process depends upon a "neutral" ion pump in the gallbladder epithelium that transports sodium, anions, and water out of the bile into the blood. Hepatic bile is thus progressively converted from a solution in which the major ions are sodium, chloride, bicarbonate, and bile acid to concentrated gallbladder bile, which consists predominantly of sodium and bile acid ions. Bile remains isosmotic with the serum during this transformation in composition. Since the osmotic coefficient of sodium bile salt is, on the average, only about 0.45 (2), the total concentration of ions in concentrated bile must be about twice that of serum. Typically, whole bile sodium and bile acid concentrations are of the order of 300 mEq per L each. During this concentrating process, changes also occur in the levels of potassium, which increase from 4 to 5 mEq per L in newly formed hepatic bile to 10 to 14 mEq per L in concentrated gallbladder bile. This increase in potassium concentration has been assumed to be due to "secretion"